The present invention relates to die casting equipment, and more particularly to shot system through which molten metal is transferred into a die.
Die casting is used to fabricate a multitude of metal articles ranging from automotive parts to children toys. A typical die casting machine includes a pair of die halves each formed with a void corresponding to a portion of the article to be cast. When the two die halves are brought together in proper alignment, their respective voids cooperate to form a die cavity corresponding to the shape of the article to be cast. Molten metal is introduced into the die cavity and allowed to cure-typically by cooling the molten metal until it solidifies. Once the metal is sufficiently cured, the die halves are opened and the cast article is removed from the die cavity. The die halves can be reclosed and the process repeated to form the desired number of identical articles.
A common method for introducing molten metal into the die cavity is to use a shot sleeve assembly including a shot sleeve and a reciprocating plunger. The shot sleeve includes a pour hole and an internal bore communicating with the die cavity. The molten metal to be injected into the die cavity is ladled through the pour hole into the sleeve. The plunger travels within the sleeve to inject or force the molten metal out of the sleeve and into the die cavity. After the "shot," the plunger is retracted to permit the sleeve to be filled for the next shot.
In a typical die casting environment, the sleeve is only partially filled with the volume of metal corresponding to the volume of the die cavity. A shot with a partially filled sleeve is called an "open shot." During an open shot, a turbulent wave forms in front of the plunger as it advances. This wave creates and entraps air bubbles within the molten metal, ultimately resulting in a porous casting.
To produce higher quality castings, "closed shot" assemblies have been developed. A closed shot sleeve has a volume corresponding to the volume of the die cavity. Consequently, the sleeve is completely filled with molten metal and the pour hole is closed before the plunger advances.
A unique and ingenious closed shot system is disclosed in U.S. Pat. No. 5,205,338 issued Apr. 27, 1993 to Shimmell. A filling cylinder intersects the shot sleeve and includes a reciprocating slide valve. After the shot sleeve is filled with molten metal, the slide valve is actuated to seal off the filling hole in the shot sleeve. Consequently, the shot sleeve is completely filled and sealed prior to the advancement of the plunger. While a significant advance in the art, the closed shot sleeve system of the '338 patent requires relatively complex machining in its manufacture. Further, the arrangement includes a number of moving parts that are subject to wear and fatigue.